The present invention relates generally to precision machines for mounting an optical element, and more particularly to a projection optical system in an exposure apparatus, etc. The present invention is suitable, for example, for a retainer that holds an optical element for a more precise imaging relationship in an exposure apparatus in projecting and exposing an image on an original sheet, such as a mask or reticle (these terms are used interchangeably in this application) onto an object, such as a single crystal substrate for a semiconductor wafer, a glass plate for a liquid crystal display (LCD). The exposure apparatus is used to fabricate a semiconductor device, an image pick-up device (such as a CCD), and a thin film magnetic head.
The fabrication of a device using the lithography technique has employed a projection exposure apparatus that uses a projection optical system to project a circuit pattern formed on a mask onto a wafer and the like, thereby transferring the circuit pattern. The projection optical system enables diffracted beams from the circuit pattern to interfere on the wafer and the like, so as to form an image.
The devices to be mounted on electronic apparatuses should be highly integrated to meet recent demands for miniaturization and low profile of electronic apparatuses, and finer circuit patterns to be transferred or higher resolution have been demanded increasingly. A short wavelength of a light source and an increased numerical aperture (“NA”) in a projection optical system are effective to the high resolution as well as a reduced aberration in the projection optical system.
An optical element, such as a lens and a mirror, when deforming in an projection optical system causes aberration because an optical path refracts before and after the deformation and light that is supposed to form an image at one point does not converge on one point. The aberration causes a positional offset and short-circuits a circuit pattern on a wafer. On the other hand, a wider pattern size to prevent short-circuiting is contradictory to a fine process. Therefore, a projection optical system with small aberration should hold its optical element(s) without changing a shape and a position relative to the optical axis of the optical element in the projection optical system so as to maximize the original optical performance of the optical element. In particular, the projection lens tends to have a larger caliber and a larger lens capacity due to the recent high NA in the projection optical system, and easily deforms by its own weight. In addition, diffraction optical elements, which have been eagerly studied recently, also tend to deform due to its thinness.
Accordingly, a conventional retainer has used a screw ring or ball pushing to compress and fix the top of an optical element that has been supported at its entire peripheral by a metal frame, or supported the optical element at three points at regular intervals in its circumferential direction.
Alternatively, Japanese Patent Application Publication No. 11-149029 proposes a retainer that supports an optical element using three optical-element support members provided at regular intervals in its circumferential direction, and elastic members that push up against the gravity the circumferential part of the optical element supported by the support members.
A metal-frame support of the entire peripheral of the optical element actually results in three point supports or contacts at irregular angles, because the metal frame that contacts the optical element has an undulated support surface unfit for the optical element's contact surface shape. As a result, this support transforms the optical element's surface, and deteriorates the optical element's optical.
On the other hand, the three points supports of the optical element at regular interval in a circumferential direction cause optical element's own weight deformations at these three points of the support members. In particular, a large aperture lens deforms significantly by its own weight, undesirably deteriorating its optical performance.
Moreover, the retainer proposed in Japanese Patent Application Publication No. 11-149029, varies forces applied to the elastic members that push up the optical element against gravity or forces directly applied to the optical element according to material's physical performance and size, and has a difficulty in precise control over the relative accuracy among the forces applied to the elastic members. Thereby, the non-uniform loads applied to respective elastic members deform the optical element.
In other words, the conventional optical-element retainers have not yet contributed to a high-resolution projection optical system with less aberration which prevents optical element's deformations which would otherwise cause deteriorated imaging performance, and meets fine processing requests.